Methods and systems for forward error correction for measurement while drilling (MWD) communication systems

ABSTRACT

A communication method is provided for a communication system comprising a transmitter and a receiver. The method involves communicating data from the transmitter to the receiver over a banded communication channel. The method comprises: applying a forward error correction, FEC, code to data to be transmitted to obtain FEC-encoded data; assigning the FEC-encoded data into a plurality of sub-channels; modulating the data from each of the plurality of sub-channels into a corresponding one of a plurality of sub-bands, the plurality of sub-bands having spaced apart center frequencies; and concurrently transmitting the data from the plurality of sub-bands onto a banded communication channel, the banded communication channel comprising one or more pass-bands and one or more stop-bands.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/331,515 filed 21 Oct. 2016 entitled METHODS AND SYSTEMS FOR FORWARDERROR CORRECTION FOR MEASUREMENT WHILE DRILLING (MWD) COMMUNICATIONSYSTEMS, which itself is a continuation of PCT application No.PCT/2015/050328 entitled METHODS AND SYSTEMS FOR FORWARD ERRORCORRECTION FOR MEASUREMENT WHILE DRILLING (MWD) COMMUNICATION SYSTEMSand filed 21 Apr. 2015, which in turn claims the benefit of the priorityof (and the benefit under 35 USC § 119 of) U.S. application No.61/982,851 entitled Methods and Systems for Forward Error Correction forMeasurement While Drilling (MWD) Communication Systems and filed 22 Apr.2014. All of the applications referred to in this paragraph are herebyincorporated herein by reference.

TECHNICAL FIELD

The invention relates to sub-surface drilling. Particular embodimentsprovide methods and systems for communication along a drill string.

BACKGROUND

Wells of the type commonly used for fossil fuel exploration andextraction and the like are often several kilometers deep. Typically,these wells or “boreholes” are drilled using pipes (often referred to as“drill strings”) assembled from sections (often referred to as “pipestands”) connected end-to-end by suitable connection joints. Pipe standsmay typically be about 30 to 45 feet long. To form a borehole, the drillstring is rotated such that a drill bit attached to its “downhole” (oroperative) end bites into the earth. Additional pipe stands aretypically added to the “uphole” (or surface) end of the drill string asthe borehole deepens.

Fluid, often referred to as “drilling mud” is typically pumped throughan axial bore in the drill string from the surface to the downhole endof the drill string. The drilling mud typically exits the drill stringat the downhole end and returns to the surface through the space betweenthe drill string and the borehole. The drilling mud may cool andlubricate the drill bit, power the drill bit (e.g. through hydrodynamicpressure), provide a deposit on the borehole wall to seal the formation,and remove debris from the borehole.

There is a general desire to communicate information from a downholelocation of the drill string (e.g. at or near the drill bit) to anuphole location (e.g. a surface location at or near the opening of theborehole). Such communication may permit monitoring of one or moresensors at the downhole location and may also permit control of thedrilling operation (e.g. steering, drilling fluid pump parameters,rotational speed and/or the like) based on feedback received from suchsensors. Such sensors (which are referred to as measurement whiledrilling (MWD) sensors) may sense characteristics of pipe string, thedrill bit and/or the borehole. Examples of MWD sensor information mayinclude temperature information, pressure information, inclineorientation information, azimuthal orientation information, vibrationinformation, drilling torque information and/or the like. In addition tosensor information, it may be desirable to communicate managementinformation from the downhole location to the uphole location. By way ofexample, such management information may include information related tothe sensor information (e.g. the amount sensor data, the type of sensordata, the transmission order of sensor data and/or the like).

One technique which has been proposed for communicating MWD informationfrom a downhole location to an uphole location involves acoustictelemetry through the drill string. The efficacy of acoustic telemetrydepends on the channel through which the acoustic signal travels. In thecase of acoustic telemetry through a drill string, the channel ortransmission medium comprises the drill string itself, which may exhibita variety of acoustic properties. In particular, because of the(typically) repetitive spacing of pipe stands and joints therebetween,there are spectral stop-bands (i.e. frequency bands of substantialattenuation) within the frequency spectrum associated with acousticcommunication. Between these stop-bands, there are pass-bands whichpermit the transmission of acoustic energy.

In some circumstances, there is no mechanism for communication down theborehole—i.e. there is no mechanism to communicate information from theuphole location to the downhole location. Without the ability tocommunicate from the uphole location to the downhole location, it can bedifficult to dynamically adapt transmission of MWD information or otherinformation from the downhole location to the uphole location on thebasis of information known only at the uphole location.

There is a general desire to communicate information from a downholelocation of a drill string (e.g. at or near the drill bit) to an upholelocation (e.g. a surface location at or near the opening of theborehole).

The foregoing examples of the related art and limitations relatedthereto are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

One aspect of the invention provides a method, in a communication systemcomprising a transmitter and a receiver, for communicating data from thetransmitter to the receiver over a banded communication channel. Themethod comprises: applying a forward error correction, FEC, code to datato be transmitted to obtain FEC-encoded data; assigning the FEC-encodeddata into a plurality of sub-channels; modulating the data from each ofthe plurality of sub-channels into a corresponding one of a plurality ofsub-bands, the plurality of sub-bands having spaced apart centerfrequencies; concurrently transmitting the data from the plurality ofsub-bands onto a banded communication channel, the banded communicationchannel comprising one or more pass-bands and one or more stop-bands.

Another aspect of the invention provides a communication systemcomprising a transmitter and a receiver for communicating data from thetransmitter to the receiver over a banded communication channel. Thetransmitter is configured to: apply a forward error correction, FEC,code to data to be transmitted to obtain FEC-encoded data; assign theFEC-encoded data into a plurality of sub-channels; modulate the datafrom each of the plurality of sub-channels into a corresponding one of aplurality of sub-bands, the plurality of sub-bands having spaced apartcenter frequencies; concurrently transmit the data from the plurality ofsub-bands onto a banded communication channel, the banded communicationchannel comprising one or more pass-bands and one or more stop-bands.The receiver may be configured to configured to: receive, at thereceiver and across the banded communication channel, the transmitteddata; and decode the transmitted data received at the receiver inaccordance with the FEC code to recover received data.

Another aspect of the invention provides a method, in a communicationsystem comprising a transmitter and a receiver, for communicating datafrom the transmitter to the receiver over a banded communicationchannel. The method comprises: assigning data to be transmitted into aplurality of sub-channels; for each sub-channel, applying a forwarderror correction, FEC, code to the data in the sub-channel to obtainFEC-encoded data; modulating the FEC-encoded data from each of theplurality of sub-channels into a corresponding one of a plurality ofsub-bands, the plurality of sub-bands having spaced apart centerfrequencies; concurrently transmitting the data from the plurality ofsub-bands onto a banded communication channel, the banded communicationchannel comprising one or more pass-bands and one or more stop-bands.

Another aspect of the invention provides a communication systemcomprising a transmitter and a receiver for communicating data from thetransmitter to the receiver over a banded communication channel. Thetransmitter is configured to: assign data to be transmitted into aplurality of sub-channels; for each sub-channel, apply a forward errorcorrection, FEC, code to the data in the sub-channel to obtainFEC-encoded data; modulate the FEC-encoded data from each of theplurality of sub-channels into a corresponding one of a plurality ofsub-bands, the plurality of sub-bands having spaced apart centerfrequencies; concurrently transmit the data from the plurality ofsub-bands onto a banded communication channel, the banded communicationchannel comprising one or more pass-bands and one or more stop-bands.

Another aspect of the invention provides a method for communicatingbetween a downhole location on a drill string and an uphole location onthe drill string. The method comprises: providing, at the downholelocation, an acoustic transmitter connected for transmitting an acousticsignal into a communication channel comprising the drill string;providing, at the uphole location, an acoustic receiver connected forreceiving a transmitted acoustic signal from the communication channel;applying a forward error correction, FEC, code to data to be transmittedto obtain FEC-encoded data; assigning the FEC-encoded data into aplurality of sub-channels; modulating the data from each of theplurality of sub-channels into a corresponding one of a plurality ofsub-bands, the plurality of sub-bands having spaced apart centerfrequencies; concurrently transmitting the data from the plurality ofsub-bands from the transmitter onto the communication channel, thecommunication channel comprising one or more pass-bands and one or morestop-bands.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than restrictive.

FIG. 1 shows a non-limiting example of how MWD information may bepackaged into a data frame according to a non-limiting exampleembodiment.

FIG. 2 schematically depicts a number of processes and system componentsin a MWD acoustic telemetry system according to a particular embodiment.

FIG. 3 is a schematic depiction showing how a FEC encoded data frame maybe transmitted across multiple sub-bands according to a particularembodiment.

FIG. 4 is a schematic depiction showing a data frame split or parsedinto several streams (sub-channels) for transmission on correspondingsub-bands with separate application of FEC encoding to each stream(sub-channel) according to a particular embodiment.

FIG. 5 is a schematic depiction showing a transmitter 54 configured tocombine FEC applied across sub-bands and within sub-bands according to aparticular embodiment.

FIG. 6 schematically depicts the modulator of a transmitter configuredto apply an orthogonal FDM (OFDM) modulation scheme, whereby select OFDMsub-carriers may be activated to transmit into desired sub-bandsaccording to a particular embodiment.

FIG. 7 is a schematic depiction of how a transmitter may set or beconfigured to set the frequencies of the sub-bands (e.g. their centerfrequencies and/or their frequency-domain widths) based on estimatedfrequency domain locations of pass-bands in the acoustic channelaccording to a particular embodiment.

FIG. 8 is a schematic depiction of how a transmitter may set or beconfigured to set the frequencies of the sub-bands (e.g. their centerfrequencies and/or their frequency-domain widths) in circumstances whereestimates of the frequency domain locations of pass-bands are not knownor not known with sufficient accuracy according to a particularembodiment.

FIG. 9 is a schematic depiction of a drill string comprising a pluralityof pipe stands and a telemetry system and corresponding communicationmethods according to a particular embodiment.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. Accordingly,the description and drawings are to be regarded in an illustrative,rather than a restrictive, sense.

One technique which has been proposed for communicating MWD informationfrom a downhole location to an uphole location involves acoustictelemetry through the drill string itself. In such circumstances, theacoustic propagation channel comprises stop bands and pass-bands. Forthe purposes of this description and the accompanying claims, apass-band may be considered to be a contiguous region of the channelspectrum (i.e. a contiguous frequency domain region) around a localmaximum of the channel magnitude response where the channel magnituderesponse is within 15 dB of the local maximum. A stop-band may beconsidered to be a region of the channel spectrum (i.e. a frequencydomain region) which is not in a pass-band. In some embodiments orapplications, the communication system may be a unidirectionalcommunications link from a transmitter at the downhole location to areceiver at the uphole location. In such unidirectional systems, theuphole receiver cannot communicate information to the downholetransmitter about the precise frequency-domain location locations ofpass-bands or stop-bands in the acoustic frequency spectrum.

FIG. 9 is a schematic depiction of a drill string 100 comprising aplurality of pipe stands 102 and a telemetry system 50 (andcorresponding communication methods) according to a particularembodiment. Telemetry system 50 of the FIG. 9 embodiment comprises atransmitter 54 which is located at a downhole location 80 (i.e. alocation relatively close to the drill bit 82) and a receiver 56 whichis located at an uphole location 84 (i.e. a location relatively far fromdrill bit 82). In the illustrated embodiment, receiver 56 is locatedabove ground 86, although this is not necessary. Transmitter 54 receivesdata from data sources (not expressly shown in FIG. 9) and communicatesthat data up drill string 100 to receiver 56. While such data sourcescould generally include any data source, it is envisaged that in someembodiments, such data sources will include information from toolsand/or sensors related to the drilling operation. In particularembodiments, transmitter 54 uses suitable transducers (e.g.electromechanical transducers—not expressly shown) to transmit anacoustic signal carrying the data along drill string 100—i.e. such thatdrill string 100 itself provides an acoustic communication channel 64between transmitter 54 and receiver 56. Receiver 56 may receive theacoustic signal from drill string 100 (i.e. acoustic channel 64) andextract the data from the data sources, such that this data is availableat uphole location 84. In some embodiments, telemetry system 50 isunidirectional in the sense that data is only communicated fromtransmitter 54 to receiver 56 (and not in reverse). This is notnecessary, however, and in some embodiments, receiver 56 may communicateinformation to downhole transmitter 56, though acoustic channel 64 orotherwise.

During drilling operations, as pipe stands 102 are added to drill string100 (or removed from drill string 100) and/or for a variety of otherreasons, the acoustic properties of drill string 100 (and thecorresponding acoustic channel 64) change over time. For example, thefrequency-domain locations of the pass-bands and/or stop-bands maychange over time. It is expected that the passbands of channel 64 willbecome narrower and may shift locations in the frequency domain as thedownhole location 80 of transmitter 54 gets further away from the upholelocation 84 of receiver 56 (e.g. as drill bit 82 gets deeper into ground86 or as pipe stands 102 are added to drill string 100).

Aspects of the invention provide for acoustic transmission of data (e.g.MWD data) from a transmitter at a downhole location to a receiver at anuphole location, wherein the transmitter codes the MWD data using aforward error correction (FEC) coding technique and transmits the FECencoded bits (coded data), possibly lacking knowledge of the spectrallocations of pass-bands in the acoustic channel. The code is configuredwith sufficient redundancy to permit the receiver to decode the receivedsignal(s) and obtain the transmitted data, even though portions of thecoded data have not been received due to transmission of such portionsinto stop bands of the channel.

In MWD acoustic telemetry from a downhole location 82 (FIG. 9) to anuphole location 84 through a drill string 100 (i.e. where thecommunication channel 64 comprises the drill string 100 itself), thechannel 64 is a banded channel having pass-bands through which data maybe communicated and stop-bands through which data is so severelyattenuated that it may not be communicated. Further, thefrequency-domain locations of the pass-bands and stop-bands of thedrill-string channel 64 may change over time. Banded channels aredistinct from fading channels. In a fading channel there is afluctuation of the signal to noise ratio (SNR) across the band ofinterest. RF coding and modulation schemes use sub-carriers andinterleavers to allocate and randomize the transmission over the bestpossible parts of the communication channel. Where the SNR is low, theerror rate may be high, and this may be overcome by transmissions athigher power or with stronger FEC. In contrast, in MWD acoustictelemetry communication systems such as telemetry system 50, the channel64 is banded, having pass-bands and stop-bands which may change overtime. Further, some MWD acoustic telemetry systems areunidirectional—i.e. communication is only transmitted from a downholetransmitter 54 to an uphole receiver 56 and not in reverse. In suchunidirectional WMD acoustic telemetry systems, there is no availablefeedback from the uphole receiver 56 to the downhole transmitter 54 toprovide transmitter 54 with knowledge or estimates of the pass-bands orstop-bands of the acoustic frequency spectrum of channel 64 and data maybe transmitted indiscriminately over acoustic channel 64 (i.e. withoutknowledge of the channel's frequency domain characteristics). Under suchcircumstances it may be inefficient to spread data across the entireband. The data allocated to stop bands is likely completely muted, andthus mere transmission at a higher power level and/or with stronger FECmay be futile.

FIG. 1 shows a non-limiting example of how MWD information 10 (includingtool (e.g. sensor) data 10A and/or management data 10B) may be packagedinto a data frame 12 according to a non-limiting example embodiment. Asimplified frame in this example comprises a 12 bit management word, a10 bit tool incline reading, a 9 bit tool azimuth reading, and a 7 bitreading of the tool vibration. In some embodiments, a frame 12 maycomprise any type of data 10 (e.g. MWD data, such as, by way ofnon-limiting example, other tool data such as temperature data, gammareadings, other types of information that would be desirably sent from adownhole location to an uphole location and/or the like). Referencesherein to data for a particular tool should be understood to includedata relating to any particular tool used in connection with thedrilling operation or data from any particular sensor used in connectionwith the drilling operation. In general, however, word 12 may comprisedata from any suitable data source. In some embodiments, the word lengthof each data element within frame 12 may be configured for other sizes(e.g. in the range 7 to 12 bits and/or the like). In some embodiments,the length of a particular frame 12 may vary. In the FIG. 1 example, theframe format and frame length is constant from frame to frame, and adata element from a tool is included once per frame. This is notnecessary. In some embodiments, some tools may provide multiple dataelements per frame and some may provide data elements once per severalframes. The content of the management word within frame 12 may containany of a variety of information, including, by way of non-limitingexample, utility and control bits, information relating to frame format,frame size, frame synchronization patterns, parity bits and/or the like.

In prior art MWD telemetry systems, the frame data is transmittedserially by means of a sequential train of mud pulses, electromagneticwaves, acoustic chirps or other acoustic pulses and is tuned to a singlepass-band at or near the mechanical resonance frequency of thetransmitter such that the system is mechanically resonant in thatpass-band. In the prior art, such serial data may be modulated byamplitude shift keying (ASK), phase shift keying (PSK) or frequencyshift keying (FSK) in attempt to overcome the frequency selectiveness orother limitations of the transmission channel. This serial transmissionyields a sequential train of modulated waves that are carried by thetransmission medium. The transmission medium may comprise drilling fluid(for mud pressure telemetry), drill formations (for electromagnetictelemetry), or drill pipe (for acoustic telemetry).

In some embodiments, the sequence of frame bits containing the MWDtool/sensor data is transmitted across several sub-bands of the acousticchannel, with Forward Error Correction applied to the frame bits. Toimprove the reliability of the telemetry communication, and to permitreliable communication from greater drill depths, Forward ErrorCorrection (FEC) is applied to the data from the data sources. ApplyingFEC to data may be referred to as FEC encoding the data. In someembodiments, the MWD data is FEC encoded before assignment tosub-channels or sub-bands to provide FEC encoding across sub-bands. FECencoding across sub-bands may provide improvements in telemetry linkswith strongly banded communications channels. Such FEC encoding acrosssub-bands can be used to mitigate uncertainty in the frequency-domainlocations of pass-bands. In some embodiments, the transmitter may haveaccess to uncertain estimates of the frequency-domain locations ofpass-bands, wherein, for example, the actual frequency-domain locationsof the pass-band centers are offset from the estimated pass-band centersby more than 10% of the frequency domain width of the pass-band. In someembodiments, this uncertainty in the estimated frequency-domainlocations of the pass-band centers is greater than 20% of the frequencydomain width of the pass-band. The application of FEC codes can help toreliably transmit and receive data over banded channels having suchuncertainties. In some embodiments, individual FEC coding schemes areadditionally or alternatively applied to the data streams (sub-channels)assigned to separate sub-bands to provide FEC encoding within sub-bands.FEC coding within sub-bands may provide improvements in telemetry linkswith noisy and/or heavily attenuated communications channels.

FIG. 2 schematically depicts a number of processes and system componentsin a MWD acoustic telemetry system 50 according to a particularembodiment. The FIG. 2 acoustic telemetry system 50, which may bedeployed in the FIG. 9 pipe string 100, is a communication system foracoustic telemetry of MWD data which comprises FEC encoding.

The FIG. 2 embodiment of the MWD telemetry system 50 comprises datasources 52 (e.g. MWD tools/sensors 52): These tools and sensors 52 maysense, measure or otherwise obtain data relevant to the drillingoperation implemented by a drill string 100 (FIG. 9) which telemetrysystem 50 uses as an acoustic channel 64. In this description,references to tools and sensors 52 are used interchangeably to refer todata sources, which provide data to a transmitter 54 of acoustictelemetry system 50. Transmitter 54 and tools/sensors 52 are typicallylocated at a downhole location 82 in the drill string 100. Downholetransmitter 54 communicates the data from tools/sensors 52 to an upholereceiver 56. As is known to those skilled in the art, system 50 maycomprise analog to digital converters (ADCs), amplifiers, other signalconditioning components and/or the like (not expressly shown in FIG. 2)which may provide data from tools/sensors 52 to transmitter 54 in aformat that is suitable for use by transmitter 54. In some embodiments,data from tools/sensors 52, once digitized, is inserted into assignedfields in a MWD transmission frame.

In conventional prior art MWD telemetry, the data in MWD frames receivedfrom tools/sensors is directly modulated and transmitted as mud pulses,mud waves, electromagnetic waves, or acoustic chirps or pulses tuned toa single frequency. In contrast to these prior art techniques, acoustictransmitter 54 of the FIG. 2 telemetry system 50 comprises an FECencoder 58 which applies FEC encoding to the data received fromtools/sensors data 52 before modulation (by modulator 60) andtransmission (by acoustic transducer(s) 62). Suitable FEC schemes whichmay be implemented by FEC encoder 58 include, without limitation,convolutional codes, turbo codes, block codes, concatenated codes, 2Dcodes and/or the like.

Transmitter 54 comprises a modulator 60 which encodes or otherwisemodulates the FEC encoded bits as waveforms for transmission. To enablesub-banded modulation (i.e. modulation of data into a plurality ofsub-bands for transmission across the drill-string channel), modulator60 may use any suitable digital modulation scheme together withfrequency division multiplexing (FDM). Non-limiting examples, ofsuitable digital modulation schemes include: phase-shift key (PSK) (e.g.binary phase shift key (BPSK), quadrature phase shift key (QPSK),differential QPSK), quadrature amplitude modulation (QAM) and/or thelike. Concurrent FDM streams may be tuned to particular sub-bands. Ifany information or estimates are known about the frequency domainlocations of pass-bands within the spectrum of the drill-string acousticchannel (FIG. 7), then the frequencies of the sub-bands (e.g. theircenter frequencies and/or their frequency-domain widths) may beconfigured based on the frequency domain locations (e.g. the centerfrequencies and/or frequency domain widths) of such pass-bands. Ifestimates are not known about the frequency-domain locations ofpass-bands in the acoustic channel, then the frequencies of thesub-bands (e.g. their center frequencies and/or their widths) may betuned such that the sub-bands are evenly spaced apart in the frequencydomain (FIG. 8). In some applications and/or embodiments, thefrequency-domain width of a sub-band may be considered to be thefrequency-domain distance between the two frequencies (above and belowthe center frequency of the sub-band) at which the transmission power isless than center frequency by 10 dB. A sub-band may be said to belimited to its frequency-domain width and two sub-bands may be describedas being spaced apart from one another in the frequency domain if theirrespective widths (or locations) are non-overlapping in the frequencydomain.

To enable sub-banded modulation (i.e. modulation of data into aplurality of sub-bands for transmission across the drill-stringchannel), modulator 60 may (in addition or in the alternative to PSK andFDM) apply an orthogonal FDM OFDM) modulation scheme, whereby selectOFDM sub-carriers may be activated to transmit into desired sub-bands(FIG. 6). In the FIG. 6 embodiment, an OFDM modulation technique mapsthe coded bits to QPSK symbols and assigns these symbols to OFDMsub-carriers across the acoustic bands. Modulator 60 may select or maybe configured to select the frequencies of the sub-bands in a mannersimilar to that discussed above in the PSK and FDM embodiment. QPSKsymbols may then be mapped to OFDM sub-carriers that are activated atthe desired sub-band frequencies.

In some embodiments, modulator 60 may apply additional up-sampling,filtering, guarding (e.g. cyclic prefix for OFDM) and D/A conversion,where suitable or desired.

Transmitter 54 comprises transmit transducer(s) 62. Transmittransducer(s) 62 may comprise one or more electro-mechanical transducers62 which convert the electrical signal (from modulator 60) incorporatingthe modulated data into a corresponding mechanical signal and impart themechanical signal on communication medium (i.e. channel) 64. In theillustrated FIG. 2 embodiment, where telemetry system 50 communicatesdata acoustically on an acoustic communication medium (i e channel) 64,such electro-mechanical transducers 62 may comprise piezo-electricand/or magnetostrictive transducers. As discussed above and in moredetail below, transmitter 54 may transmit data into several concurrentsub-bands. The transmission of data onto several concurrent sub-bandsmay be accomplished by one wide-band transducer 62 (which may itselfcomprise a plurality of transducer elements) or by several tunednarrow-band transducers 62 (each of which may comprise a plurality oftransducer elements).

Transmit transducers 62 impart a mechanical (e.g. acoustic) signal oncommunication medium 64. In some embodiments which employ acoustictransmission, the telemetry medium (i e channel) 64 comprises the pipeof the drill string 100 itself, which conveys acoustic waves fromdownhole transmitter 54 to an uphole receiver 56 located further updrill string 100.

Receiver 56 comprises one or more receive transducers 66 (e.g.electro-mechanical transducers, such as piezo-electric transducersand/or accelerometers; optical transducers, such as interferometers;and/or any other suitable transducer(s)). Receive transducers 66 convertthe acoustic signal received from channel 64 to an electrical signal forprocessing by demodulator 68. As is known in the art, receiver 56 maycomprise suitable signal conditioning electronics (e.g. filters,amplifiers, analog to digital converters (ADCs) and/or the like; notexpressly shown) which may condition the signal received from receivetransducers 66 to provide a signal in a suitable format for use bydemodulator 68. Demodulator 68 (or other signal conditioningcomponent(s) (not shown)) of receiver 56 may further process thereceived signal by synchronization (acquisition, tracking) andequalization prior to demodulation. In the case of the illustrated FIG.2 embodiment, demodulator 68 outputs received data bits that are stillFEC encoded.

Receiver 56 also comprises an FEC decoder 70. FEC decoder 70 recoversthe MWD data bits from FEC encoded bits output from demodulator 68.Where applicable, FEC decoder 70 may also correct errored bits. By wayof non-limiting example, such errors may be caused by drilling noise orpump noise. Where applicable, FEC decoder 70 may also perform thefunction of filling in muted bits (or erasures). Coded bits may be mutedif they are modulated (by transmitter 54) into stop-bands ofcommunications channel 64, for example. Recovery of muted bits may beaccomplished by a decoder 70 capable of erasure processing.

In the illustrated embodiment, system 50 comprises optional MWD reports72 which may display, store or otherwise provide tool/sensor data foruse by the MWD operator and/or for other suitable uses or users.

Aspects of the invention may comprise various combinations orsub-combinations of one or more of the elements of acoustic telemetrysystem 50 including suitably configured hardware, software and/orprocess/method steps.

FIG. 3 is a schematic depiction showing how a FEC encoded data frame maybe transmitted (e.g. by transmitter 54) across multiple sub-bandsaccording to a particular embodiment. In some applications orembodiments, transmitter 54 has no (or limited) estimates of thefrequency-domain locations of pass-bands of acoustic channel 64 (e.g.because such estimates cannot be communicated to downhole transmitter54). For example, in some embodiments or applications, communicationsystem 50 is unidirectional and there is no telemetry feedback (i.e.communication) from uphole receiver 56 which may be located at upholelocation 84 (e.g. on the drill deck) back down to downhole transmitter54 (which may be located at downhole location 80 at or near drill bit82) to provide transmitter 54 with information or estimates as to thefrequency-domain locations of the pass-bands of acoustic channel 64. Insome such embodiments, transmitter 54 may indiscriminately transmit datainto a plurality of sub-bands across the spectral width of acousticchannel 64, but receiver 56 may only receive the spectral components ofthe transmitted signal that are transmitted into pass-bands of acousticchannel 64. The spectral components of the transmitted signal that aretransmitted into the stop-bands of acoustic channel 64 are attenuated.As a result, coded bits that are transmitted into the stop bands aremuted and may be difficult or even impossible to decode.

In the FIG. 3 embodiment, a FEC code (e.g. a Reed Solomon (RS) blockcode or some other suitable type of FEC code) is applied to incomingdata 10 (which may be provided in the form of data frames 12). Theapplication of this FEC code may be implemented by FEC encoder 58. Inthe FIG. 3 embodiment, the FEC coded bits 14 are then concurrentlytransmitted across a plurality of sub-bands. Such sub-bands arerepresented in FIG. 3 by their center frequencies f0, f1, f2, f3. In theFIG. 3 embodiment, there are four illustrated sub-bands, but there maygenerally be any suitable number of sub-bands. The sub-bands may betransmitted into (or occupy) a portion of the acoustic spectrum ofchannel 64, where it is considered that pass-bands are likely to befound throughout the drilling operation.

In some applications or embodiments, transmitter 54 may have someknowledge or estimates of the frequency-domain locations of thepass-bands of channel 64 (e.g. where such information can be obtainedfrom an uphole receiver 56, can be empirically determined and/or thelike). In such applications or embodiments, transmitter 54 may configurethe sub-bands (e.g. the center frequencies f0, f1, f2, f3 of thesub-bands and/or the frequency-domain widths of the sub-bands) based onsuch estimates of the frequency-domain locations of the pass-bands ofchannel 64. In some embodiments, transmitter 54 may configure (or beconfigured to set) the center frequencies f0, f1, f2, f3 of thesub-bands and/or the frequency-domain widths of the sub-bands such thatthe sub-bands are located primarily within the estimated frequencydomain locations of the pass-bands. This configuration is shown in theFIG. 7 example, where, for the most part, the sub-bands (depicted ashatched rectangles) are located in pass-bands |H(f)|. In some cases, theestimates of the pass-bands locations are not accurate, in which casesome energy in a sub-band is actually transmitted into a stop-band. Inthe FIG. 7 example, this is the case for the sub-band centered at f2. Insuch cases, the FEC applied to the data by transmitter 54 may enable therecovery of the data at receiver 56.

In some embodiments, transmitter 54 may be configured to take advantageof the FEC by setting the center frequencies f0, f1, f2, f3 of thesub-bands and/or the frequency-domain widths of the sub-bands such thatthe sub-bands have frequency domain widths that are greater than thefrequency-domain widths of corresponding pass-band estimates. Forexample, in some embodiments, the sub-bands may be configured to havefrequency domain widths that are 5% greater than estimated pass-bandwidths. In some embodiments, this percentage may be 10% or 15%. Suchembodiments may take advantage of the FEC and the inaccuracy of theestimated frequency-domain locations of pass-bands. If the estimatedfrequency-domain location of a pass-band underestimates the pass-band'sactual frequency-domain width (i.e. the pass-band is actually wider thanthe estimate), then such embodiments take advantage of this inaccuracyby exploiting the entire width of the sub-band (i.e. a sub-band widthgreater than the estimated pass-band width, but possibly less than theactual pass-band width). On the other hand, if the estimatedfrequency-domain location of the pass-band overestimates the pass-band'sfrequency-domain width (i.e. the pass-band is actually narrower than theestimate), then such embodiments take advantage of the FEC to recoverdata that is transmitted into a stop band. In some embodiments, thesub-band frequency domain widths may be fixed and such widths may be setto be greater (e.g. 5%, 10% or 15%) greater than the expected worst case(narrowest) pass-band estimate.

In some applications or embodiments, the frequency-domain locations ofthe pass-bands of channel 64 cannot be precisely determined (e.g. wherefeedback from an uphole receiver is not available) or may vary as thedrilling operation progresses and the drill string lengthens orshortens. If estimates are not known about the frequency-domainlocations of pass-bands in the acoustic channel, then the frequencies ofthe sub-bands (e.g. their center frequencies and/or their widths) may betuned such that the sub-bands are evenly spaced apart in the frequencydomain (FIG. 8).

In one particular exemplary embodiment, FEC encoder 58 applies an RSblock code which may be employed across the sub-bands used bytransmitter 54 and which may be specified as follows:

-   -   (i) A RS codeword based on a GF(256) Galois Field, with N=255        and K=51 (where N is the size of the code and K is the number of        data words) is formed from 8*K=408 data bits received from a        data source (e.g. from a MWD tool(s) and/or sensor(s)). The RS        codeword comprises 8*N=2040 coded bits.    -   (ii) An OFDM modulator with 8192 sub-carriers and with sampling        rate f_(s)=20.48 KHz is applied. The acoustic channel from about        500 Hz to about 9 KHz can thus be subdivided into 3600 active        OFDM sub-carriers. Of these active sub-carriers, 540        sub-carriers may be used as pilots for synchronization leaving        3060 sub-carriers for coded bits. With a QPSK sub-carrier        modulation, 2 coded bits are modulated per subcarrier, and thus        there are 6120 coded bits in one OFDM symbol.    -   (iii) The modulated bits of an OFDM symbol may comprise the        compounding of 3 RS codewords, for a total of 3*N*8=6120 coded        bits. Each RS codeword is thus contained within an OFDM symbol        and crosses a portion (in the present example, ⅓) of the        sub-carriers.

At receiver 56 (FIG. 2), a RS decoder (e.g. FEC decoder 70) with erasurecapabilities can correct up to e=N−K erasures. By way of example, if thestop bands of channel 64 occupy at most 80% of the frequency domain spanof the sub-carriers corresponding to each RS codeword, then there willbe at most 80% erasures per RS codeword. Accordingly, in this exampleembodiment, if at least 20% of the OFDM sub-carriers corresponding toeach RS codeword can be received through the pass-bands of channel 64,then the 408 data bits from the data source can be recovered for each RScodeword, even if the locations of the stop-bands and pass-bands arecompletely unknown to transmitter 54.

The pass-band density over a frequency-domain span of interest inchannel 64 may be considered to be the ratio of the sum of the spectral(frequency-domain) widths of the pass-bands to the frequency-domainwidth of the span of interest. The stop-band density may be consideredto be ratio of the sum of the spectral (frequency-domain) widths of thestop-bands to the frequency-domain width of the span of interest. Itwill be appreciated that the sum of the stop-band density and pass-banddensity over a frequency-domain span of interest is unity. In the aboveexample, the pass band density over the frequency-domain spancorresponding to the sub-carriers of one RS codeword was assumed to be20%. By choosing a smaller K parameter (e.g. a K less than K=51 used inthe present example), some margin may be provided against downwardvariations of the pass-band density that is expected in a specific drillstring for which the RS cross code is parameterized.

In some embodiments, erasures can be detected by receive powermeasurements in the sub-bands or in individual sub-carriers received atreceiver 56. For example, if the received sub-carrier power is below aconfigurable threshold, then the 8-bit Galois Field (GF) symbolsassigned to that sub-carrier may be marked as erasures. Similarly, ifthe received power in a FDM sub-band is below a configurable threshold,then the coded bits corresponding to that sub-band may be marked aserasures. Alternately, or in combination with receive powermeasurements, in some embodiments, receiver 56 can use estimated OFDMsub-carrier Signal-to-Interference-and-Noise Ratio (SINR) to flagerasures. Methods to estimate the SINR include measuring the spread ofreceived constellation symbols, where a higher spread is indicative of alower SINR. Sub-carriers with lower SINR values are better candidatesfor having their corresponding symbols be flagged as erasures. In theconventional (single carrier) FDM case, if the SINR estimate is below aconfigurable threshold, then all of the coded bits in the correspondingsub-band may be marked as erasures. Other measures of signal quality(e.g. conventional signal to noise ratio (SNR) and/or the like) can beused in addition to or in the alternative to SINR.

In addition or alternative to the erasure-handling capabilities of theRS code, receiver 56 may also use the error detection capabilities ofthe RS code. The RS block code is a t-error correcting code withN−K=2*t. Thus, without knowledge of the erasures or with some inaccuracyin the determination of erasures, it remains possible to successfullyextract accurate data from the codeword, despite the circumstance whereparts of the codeword are incorrectly received due, for example, tostop-band attenuations. With N=255 and K=51, t=102 GF(256) symbol errorscan be detected and corrected. This corresponds to a stop-band densityof 102/256=40%.

In some embodiments, receiver 56 may use a combination of the erasureand error-handling capabilities of the RS code. A RS decoder (e.g. FECdecoder 70) with erasure-handling capabilities can correct up to e=N−Kerasures. If fewer than e OFDM subcarriers are flagged as erasures, theRS decoder will be capable of detecting and correcting errors that werenot flagged as erasures. As a non-limiting example, if at most e/2 OFDMsubcarriers are flagged as erasures, then the RS decoder can detect upto t/2 errors, where t=(N−K)/2. In some embodiments, the FEC code canaccommodate a percentage erasure rate and a percentage of thetransmitted data flagged as erasures (i.e. a flagged erasure percentage)may be less than percentage erasure rate that can be accommodated by theFEC code, thereby permitting the FEC code to accommodate at least someadditional errors. In some embodiments, a ratio of the flagged erasurepercentage to the percentage erasure rate that can be accommodated bythe FEC code is less than 80%.

In some embodiments, FEC block codes (other than RS block codes) may beused in addition to as an alternative to RS block codes. Non-limitingexamples of such block codes include Bose-Chadhuri-Hocquenghem (BCH)block codes and/or the like. In some embodiments, the parameters of theblock code (e.g. N, K etc. of the RS code) may be configurable. In someembodiments, convolutional FEC code techniques may be employed.

In some embodiments, such as in the example embodiment discussed abovewith reference to FIG. 3, transmitter 54 may transmit the coded bitsfrom a single code word output from FEC encoder 58 across the entireacoustic band over which transmitter 54 transmits data (e.g. over all ofthe sub-bands into which transmitter 54 transmits data), or across atleast a plurality of sub-bands from among the sub-bands into whichtransmitter 54 transmits data. Such embodiments, may be used, forexample, in circumstances where the spectral (frequency-domain)locations of the pass-bands and/or stop bands are not known totransmitter 54 or are not known to transmitter 54 with sufficientconfidence. Such a spectrally diverse distribution of coded bits from asingle code word (e.g. across sub-bands), may be referred to as a crosscode. In some embodiments, when the frequency-domain locations ofpass-bands of channel 64 (or estimates thereof) are known (or at leastapproximately known) to transmitter 54, transmitter 54 may transmit thecoded bits from a single code word at the output of the FEC encoder 58into one or more distinct sub-bands which are tuned to one or morecorresponding pass-bands of channel 64.

FIG. 4 is a schematic depiction showing a data frame split or parsedinto several streams (e.g. sub-channels) for transmission (e.g. bytransmitter 54) on corresponding sub-bands with separate application ofFEC encoding to each stream (sub-channel) according to a particularembodiment. In the FIG. 4 embodiment, data from within each frame 12 issplit into streams (also referred to as sub-channels), and each streamis assigned to a corresponding sub-band. In the FIG. 4 exampleembodiment, there are four streams, but, in general, transmitter 54 maybe configured to use any suitable number of streams. In the FIG. 4embodiment, an individual in-stream FEC is applied to each stream (e.g.by in-stream FEC encoders 58A-58D, which are shown as distinct in FIG. 4for explanatory purposes, but which may be separately implemented orimplemented in combination). Transmitter 54 then transmits the FECencoded bits of each stream 16A-16D over a corresponding sub-band. Insome embodiments, splitting data into streams may be accomplished attransmitter 54 by extracting tool/sensor-specific readings from an MWDdata frame presented to transmitter 54 by tools and/or sensors 52. Theseparated tool/sensor readings may be assigned to separate streams ofdata. In some embodiments, each data frame 12 may be subdivided intosub-frames, without consideration of the specific location of thetool/sensor readings within the frame, and the sub-frames may beassigned to separate streams.

In drill strings where there are several sub-bands available, thein-stream FECs 58A-58D applied to each stream may operate independentlyfrom each other and may be implemented concurrently. In someembodiments, where transmitter 54 may have some knowledge or estimatesof the frequency-domain locations of pass-bands within acoustic channel54, the sub-bands may be configured to match (at least approximately)the pass-bands of the channel. An example of such a modulation is shownin FIG. 7. In some such embodiments, transmitter 54 may set (or beconfigured to set) the center frequencies f0, f1, f2, f3 of thesub-bands to coincide at least approximately with the centers of theestimated pass-band frequency-domain locations and transmitter 54 mayset (or be configured to set) the frequency-domain widths of thesub-bands to fit within the estimated pass-band frequency-domainlocations. In applications or embodiments where the pass-bands are notknown, it may be beneficial to alternate the stream assignments (e.g. ina round robin scheme or some other variable assignment scheme) so thattool/sensor data alternates between sub-bands. In some embodiments, suchas (by way of non-limiting example) where data is split according tosensor/tool, such alternation may permit different tool/sensor data toalternate between different parts of the acoustic spectrum of channel64. An example of such a modulation is shown in FIG. 8.

In some embodiments, transmitter 54 may comprise an interleaver whichmay be used by transmitter 54 to spread consecutive data bits across theacoustic spectrum over channel 64 and over time. Using an interleaverbenefits the performance of convolutional codes, since the correspondingde-interleaver at receiver 56 may break up long strings of errored bitsthat were transmitted through adjacent sub-carriers into wide stopbands. In some applications or embodiments, where transmitter 54 mayhave some knowledge or estimates of the frequency-domain locations ofpass-bands of acoustic channel 64, it may be beneficial to interleaveover time and within pass-bands to help avoid mix-in bits fromstop-bands.

In some embodiments, transmitter 54 may employ a combination of the FIG.3 technique for applying FEC across sub-bands and the FIG. 4 techniquefor applying FEC within sub-bands. FIG. 5 is a schematic depictionshowing a transmitter 54 configured to combine FEC applied acrosssub-bands and within sub-bands according to a particular embodiment. Insome embodiments, a combination of these two FEC schemes may be appliedto provide a two-dimensional (2D) FEC encoding which comprises:

-   -   (i) Determining the sub-bands over which to transmit. In some        embodiments, where estimates of the frequency-domain locations        of pass-bands are known, determining the sub-bands over which to        transmit may comprise spectrally locating the sub-bands, as best        as may be possible, in the estimate pass-bands of acoustic        channel 64 (see FIG. 7). In some embodiments or applications,        such estimates of pass-band frequency domain locations are not        known and determining the sub-bands may comprise determining the        sub-bands to be centered at regular intervals and/or to have        common widths (see FIG. 8) or otherwise selecting sub-bands.    -   (ii) Applying a cross code to the data (using the technique        discussed above with reference to FIG. 3) prior to parsing the        data into streams, thus producing coded bits. This cross-coding        step is performed, in the FIG. 5 example embodiment, by FEC        encoder 58.    -   (iii) Subdividing or parsing the coded bits from the cross code        into streams (sub-channels), and applying an individual        in-stream FEC to each stream (sub-channel). In the illustrated        embodiment of FIG. 5, four streams are shown, but it will be        appreciated that other suitable numbers of streams could be        used. In the FIG. 5 example embodiment, the in-stream FEC        encoding step is performed by in-stream FEC encoders 58A-58D,        which may be separately implemented or implemented a single        unit. In some embodiments, the in-stream FEC scheme employed for        each sub-band stream may comprise: convolutional codes, turbo        codes, block codes and/or the like and/or a concatenation of        such codes. With this technique, each sub-band stream 18A-18D        comprises a portion of a first cross code word, a portion of a        second cross code word, etc.    -   (iv) Modulating and transmitting the coded bits from each        sub-band stream 18A-18D into a corresponding acoustic sub-band.

It will be appreciated that this FIG. 5 technique comprises forming a 2Dcode.

At the receiver end, receiver 56 may first decode the individualsub-bands using a sub-band decoder (i.e. to decode the correspondingin-stream encoding) and, subsequently, apply the output from thesub-band decoder to a cross code decoder to decode the cross code. Insome embodiments, the output from the sub-band (in-stream) decoder maybe used to flag erasures for the cross-code decoder. By way ofnon-limiting example, a Soft Output Viterbi Algorithm (SOYA) may beapplied to decode a sub-band convolutional inner code, and the softoutput may be used to generate erasure flags for a Reed Solomon or otherblock-type FEC cross-code outer code. For example, where the SOYA softoutput for a particular sub-band or sub-carrier is within some thresholdregion of uncertainty, then the coded bits corresponding to thatsub-band or sub-carrier may be flagged as being erasures. In someembodiments, the receive power level in each sub-band or sub-carrier maybe subjected to a suitable thresholding process or the like and, if thepower level does not meet this threshold, the coded bits assigned to thesub-band or carrier may be marked or flagged as erasures. Alternatively,or in suitable combination, the estimated sub-band or sub-carrier SNRmeasured as (or estimated on the basis of) the spread of the recoveredconstellation symbol may be used to mark or flag the coded bitscorresponding to the sub-band or sub-carrier as erasures.

When both the cross code and the sub-band (in-stream) code are blockcodes, iterative decoding schemes may be applied. For example, in someembodiments, such an iterative decoding scheme may comprise: first,decoding one dimension (e.g. one of the sub-band (in-stream) codes andthe cross-code) to reduce the number of bit errors; and then decodingthe second dimension (e.g. the other one of the sub-band (in-stream)codes and the cross-code) to further reduce the number of bit errors.Then the data with reduced number of errors may decoded along the firstdimension again, followed by the second dimension again. This may beiterated several times.

In some embodiments, it may be advantageous to mix codes of differentstrength within a transmission scheme.

At large depths (e.g. where transmitter 54 is relatively far downhole ascompared to receiver 56), the received acoustical signal may berelatively weak. With low receive signal power (e.g. below 0 dB), eventhe strongest codes cannot provide enough coding gain to yield asatisfactory bit error rate of better than 1%. In some embodiments,where it is known that a transmission may occur from a transmitter 54located at relatively large depths (e.g. relatively far downhole fromthe uphole location of receiver 56), it may be advantageous to applyrepetition coding, where data is repeated so that receiver 56 may applya soft combiner, such as a maximum ratio combiner (MRC), to the repeateddata. This yields SNR gains of up to 3 dB per doubling of thetransmission. Transmitter 54 can be configured to transmit MWD data withFEC applied (according to any of the techniques described herein), andalso with repetition applied.

In such embodiments, when a transmission occurs from relatively smalldepths (e.g. where transmitter 54 is relatively close to receiver 56),the SNR at receiver 56 may be sufficient, in which case FEC decoder 70may decode the first of the repeated transmissions to extract the datarelatively quickly. However, when a transmission occurs from arelatively large depth (e.g. where transmitter 54 is relatively fardownhole from receiver 56) and the SNR at receiver 56 may beinsufficient for FEC alone to recover the data, the repeatedtransmissions may be soft-combined before FEC decoder 70 attempts todecode the data. It will be appreciated that the depth of transmitter 54(relative to receiver 56) is a characteristic that can be detected orotherwise known at transmitter 54 without requiring communication ofthis information from the uphole receiver 56. Thus transmitter 54 canoperate with and without repeat transmissions depending on its depthrelative to receiver 56. Whether repetition is enabled at transmitter 54or not may be communicated to the receiver in modem management overheaddata.

In some embodiments, transmitter 54 may permanently operate withrepetition, thus providing a choice at receiver 56: stronger signalsfrom lesser depths may be decoded with less latency, and weaker signalsfrom greater depths may be decoded with larger latency. The receivepower level in a sub-band may be measured and used to determine whethersoft combining should be applied or not. Alternatively, a FEC decoderfail indicator may be used at receiver 56, whereby if FEC decoder 70indicates the failed decoding of a codeword, then decoding isre-attempted after additional soft combining. Additionally oralternatively, a parity check (e.g. CRC—Cyclic Redundancy Check) that ispart of the coded data may be calculated and checked at receiver 56,and, if it fails, the decoding is re-attempted after additional softcombining.

In some embodiments, punctured codes may be applied by transmitter 54.When a punctured code is applied, it is possible to performretransmissions of a code word while applying alternate puncturing of asame base code word. At receiver 56, the transmissions with alternatepuncturing may be blended to effectively yield a stronger code.

During transmissions from lesser depths (i.e. where transmitter 54 isrelatively close to receiver 56) and the SNR at receiver 56 is stillsufficient to decode the punctured FEC codes, FEC decoder 70 may decodethe first of the punctured transmissions to extract the data relativelyquickly. However, when a transmission occurs from a relatively largedepth (e.g. where transmitter 54 is relatively far from receiver 56) andthe SNR at receiver 56 may be insufficient for recovery of a puncturedFEC code, the receptions with alternate puncturings may be blendedbefore FEC decoding. Transmitting with a combination of differentpuncturing provides a choice at receiver 56: at lesser depths to decodewith lower latency, and at greater depths to decode heavily attenuatedsignals with a stronger FEC but at the expense of latency.

It will be appreciated that the depth of transmitter 54 (relative to thereceiver 56) is a characteristic that can be detected or otherwise knownat transmitter 54 without requiring communication of this informationfrom uphole receiver 56. Thus, transmitter 54 can operate with differentcode rates depending on its depth. The code type, code rate and type ofpuncturing may be communicated to receiver 56 in modem managementoverhead data.

In some embodiments, transmitter 54 may permanently operate with severalcode types, rates and puncturing, thus providing a choice at receiver56: stronger signals from lesser depths are decoded with less latency,and weaker signals from greater depths are decoded with larger latency.

While a number of exemplary aspects and embodiments are discussedherein, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. For example:

-   -   In some of the embodiments described above, particular exemplary        FEC codes (e.g. RS codes) are used for the purposes of        explanation. In general, however, other types of FEC coding        schemes can additionally or alternatively be used in each such        example.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

What is claimed is:
 1. A method, in a communication system comprising atransmitter and a receiver, for communicating data from the transmitterto the receiver over a banded communication channel, the methodcomprising: applying a forward error correction, FEC, code to data to betransmitted to obtain FEC-encoded data; assigning the FEC-encoded datainto a plurality of sub-channels, each sub-channel comprising aplurality of sub-carriers, each sub-carrier spaced apart in thefrequency domain; modulating the FEC-encoded data from each of theplurality of sub-channels into a corresponding one of a plurality offrequency sub-bands, the plurality of frequency sub-bands having spacedapart center frequencies, each frequency sub-band spread over a finitefrequency range surrounding its center frequency and the modulated datafor each sub-channel occupying the finite frequency range of itscorresponding frequency sub-band; and concurrently transmitting themodulated data from the plurality of frequency sub-bands onto a bandedcommunication channel, the banded communication channel comprising oneor more pass-bands and one or more stop-bands.
 2. The method accordingto claim 1 wherein concurrently transmitting the modulated data from theplurality of sub-bands onto the banded communication channel comprisestransmitting acoustic energy over the channel.
 3. The method accordingto claim 2 wherein transmitting acoustic energy over the channelcomprises transmitting acoustic energy over a drill string.
 4. Themethod according to claim 1 wherein the FEC code can accommodate apercentage error rate and wherein a stop-band density over afrequency-domain range of each sub-band is less than the percentageerror rate.
 5. The method according to claim 1 comprising: receiving, atthe receiver and across the banded communication channel, thetransmitted data; and decoding the transmitted data received at thereceiver in accordance with the FEC code to recover received data. 6.The method according to claim 1 comprising encoding the data to betransmitted according to a two-dimensional FEC encoding scheme, whereinencoding the data to be transmitted according to a two-dimensional FECencoding scheme comprises, after assigning the FEC-encoded data into theplurality of sub-channels and prior to modulating the FEC-encoded datafrom each of the plurality of sub-channels, applying, to the FEC-encodeddata assigned into each of the plurality of sub-channels, acorresponding in-stream FEC code.
 7. The method according to claim 6comprising: receiving, at the receiver and across the bandedcommunication channel, the transmitted data; and for each sub-band,decoding the transmitted data received at the receiver in the sub-bandin accordance with its corresponding in-stream FEC code to obtainsub-band decoded data; and decoding the sub-band decoded data inaccordance with the FEC code to recover received data.
 8. The methodaccording to claim 1 wherein modulating the FEC-encoded data from eachof the plurality of sub-channels into its corresponding one of theplurality of sub-bands comprises using an orthogonal frequency divisionmultiplexing (OFDM) scheme wherein, for each sub-channel, a plurality ofbits from the sub-channel are modulated concurrently onto orthogonalacoustic sub-carriers having sub-carrier frequencies within the sub-bandcorresponding to the sub-channel.
 9. The method according to claim 8comprising: receiving, at the receiver and across the bandedcommunication channel, the transmitted data; and decoding thetransmitted data received at the receiver in accordance with the FECcode to recover received data.
 10. The method according to claim 8wherein using the OFDM scheme comprises applying, to the FEC-encodeddata assigned to each of the plurality of sub-carriers, a correspondingsub-carrier FEC code.
 11. The method according to claim 10 comprising:receiving, at the receiver and across the banded communication channel,the transmitted data; and for each sub-carrier, decoding the transmitteddata received at the receiver in association with the sub-carrier inaccordance with its corresponding sub-carrier FEC code to obtainsub-carrier decoded data; and decoding the sub-carrier decoded data inaccordance with the FEC code to recover received data.
 12. The methodaccording to claim 1 wherein modulating the FEC-encoded data from eachof the plurality of sub-channels into its corresponding one of theplurality of sub-bands comprises assigning the center frequency of eachsub-band to be within an estimated frequency-domain location of acorresponding pass-band in the channel.
 13. The method according toclaim 2 comprising receiving, from the receiver, the estimatedfrequency-domain locations of the pass-bands in the channel.
 14. Themethod according to claim 1 wherein the sub-bands spread over distinctand non-overlapping ranges of frequencies.
 15. The method according toclaim 1 wherein the center frequencies of the sub-bands are evenlyspaced apart.
 16. The method according to claim 1 wherein assigning theFEC-encoded data into a plurality of sub-channels comprises assigningthe same FEC-encoded data into each of the plurality of sub-channels.17. The method according to claim 1 wherein assigning the FEC-encodeddata into a plurality of sub-channels comprises splitting theFEC-encoded data into a plurality of streams corresponding to theplurality of sub-channels and assigning each of the streams into theeach of the corresponding plurality of sub-channels.
 18. A communicationsystem comprising a transmitter and a receiver for communicating datafrom the transmitter to the receiver over a banded communicationchannel, the communication system comprising: a transmitter configuredto: apply a forward error correction, FEC, code to data to betransmitted to obtain FEC-encoded data; assign the FEC-encoded data intoa plurality of sub-channels, each sub-channel comprising a plurality ofsub-carriers, each sub-carrier spaced apart in the frequency domain;modulate the FEC-encoded data from each of the plurality of sub-channelsinto a corresponding one of a plurality of frequency sub-bands, theplurality of frequency sub-bands having spaced apart center frequencies,each frequency sub-band spread over a finite frequency range surroundingits center frequency and the modulated data for each sub-channeloccupying the finite frequency range of its corresponding frequencysub-band; concurrently transmit the modulated data from the plurality offrequency sub-bands onto a banded communication channel, the bandedcommunication channel comprising one or more pass-bands and one or morestop-bands.
 19. The system according to claim 18 comprising: a receiverconfigured to: receive, at the receiver and across the bandedcommunication channel, the transmitted data; and decode the transmitteddata received at the receiver in accordance with the FEC code to recoverreceived data.
 20. A method for providing communication between adownhole location on a drill string and an uphole location on the drillstring, the method comprising: providing, at the downhole location, anacoustic transmitter connected for transmitting an acoustic signal intoa communication channel comprising the drill string; providing, at theuphole location, an acoustic receiver connected for receiving atransmitted acoustic signal from the communication channel; applying aforward error correction, FEC, code to data to be transmitted to obtainFEC-encoded data; assigning the FEC-encoded data into a plurality ofsub-channels, each sub-channel comprising a plurality of sub-carriers,each sub-carrier spaced apart in the frequency domain; modulating theFEC-encoded data from each of the plurality of sub-channels into acorresponding one of a plurality of frequency sub-bands, the pluralityof frequency sub-bands having spaced apart center frequencies, eachfrequency sub-band spread over a finite frequency range surrounding itscenter frequency and the modulated data for each sub-channel occupyingthe finite frequency range of its corresponding frequency sub-band; andconcurrently transmitting the modulated data from the plurality offrequency sub-bands from the transmitter onto the communication channel,the communication channel comprising one or more pass-bands and one ormore stop-bands.
 21. A method according to claim 20 wherein modulatingthe data from each of the plurality of sub-channels into itscorresponding one of the plurality of sub-bands comprises using anorthogonal frequency division multiplexing (OFDM) scheme wherein, foreach sub-channel, a plurality of bits from the sub-channel are modulatedconcurrently onto orthogonal acoustic sub-carriers having sub-carrierfrequencies within the sub-band corresponding to the sub-channel andwherein the method further comprises, for each sub-carrier, flagging thetransmitted data received in association with the sub-carrier aserasures for handling during decoding based at least in part onmeasurements of receive signal power in the sub-carrier.
 22. A methodaccording to claim 20 wherein modulating the data from each of theplurality of sub-channels into its corresponding one of the plurality ofsub-bands comprises selecting orthogonal acoustic sub-carriers havingsub-carrier frequencies within an estimated frequency-domain location ofa corresponding pass-band in the channel.
 23. A method according toclaim 20 wherein modulating the data from each of the plurality ofsub-channels into its corresponding one of the plurality of sub-bandscomprises using an orthogonal frequency division multiplexing (OFDM)scheme wherein, for each sub-channel, a plurality of bits from thesub-channel are modulated concurrently onto orthogonal acousticsub-carriers having sub-carrier frequencies within the sub-bandcorresponding to the sub-channel; and wherein the method furthercomprises, for each sub-carrier, flagging the transmitted data receivedin association with the sub-carrier as erasures for handling duringdecoding of the sub-carrier decoded data based at least in part onmeasurements of receive signal power in the sub-carrier.